The Global Carbon Cycle

Chemistry

1 CARBON ON EARTH

A TOUCH OF MAGIC

The carbon cycle of the Earth differs from the other topics covered in the Princeton Primer series on the Climate of the Earth in that it is alive. Hurricanes, El Niño, and radiation in the atmosphere are all topics that one could spend a satisfying lifetime studying, but living matter somehow transcends the reductionist physical sciences that capture those other phenomena so well.

The second law of thermodynamics states that the universe runs downhill from order to disorder. Even in a universe with life in it, the principle of entropy—the drive from order to disorder—holds. But within such a universe, subject to the second law of thermodynamics, life creates for itself pockets of the most exquisite order and stability. It does so by creating even greater disorder in its surroundings, thereby adhering to the letter of the second law while giving the impression of somehow flouting it.

Living systems are able to create pockets of stability in part because they are immensely complicated. Birds and bees and flowers and trees are physical-chemical machines more intricate than any created by the human intellect. Chemical concentrations are altered using enzymes, each a chemical catalyst specifically tuned to its function, its activity regulated by means of other chemical signals. The whole machine can be replicated from a few molecular tools and the information coded in DNA. The machine works with such accuracy and subtlety that a boy can grow up to look like his father.

Another characteristic of life on Earth is that it seems to possess an intrinsic stability. Fossil carbon energy production and carbon dioxide (CO2) release take place in the context of three grand stabilizing feedback systems in the global biosphere carbon cycle: the weathering CO2 thermostat, the oxygen homeostat, and the ocean's calcium carbonate pH-stat.

Carbon Dioxide

Earth's climate has navigated a path that stayed within a narrow range, the freezing and boiling points of water, since the very first sedimentary rocks appeared, shortly after the birth of the Earth. Meanwhile, the heat source for the surface of the Earth, the sun, has gotten 25% brighter over geologic time. This faint young sun paradox was first noted by the astronomer Carl Sagan (Sagan and Mullen 1972).

Part of the explanation has to do with the carbon cycle and a mechanism called the weathering CO2 thermostat, which stabilizes Earth's climate by regulating the CO2 concentration in the atmosphere (see chapter 2). The idea is that the rate of chemical weathering, which results in carbon sequestration as calcium carbonate (CaCO3) in sedimentary rocks, increases as a function of temperature, pulling the CO2 concentration down if the Earth is too warm or allowing it to build up if the Earth is too cold. Unfortunately for the global warming climate event, it will take hundreds of thousands of years for the thermostat to restore Earth's climate to its natural carbon-cycle balance (Archer 2009).

Box 1.1

Units of the Carbon Cycle

The science of the carbon cycle is fraught with unit wars, analogous to metric versus English units at the grocery store or the gas station. One learns one's way around in one set of units, and information presented in other units has to be translated to understand it. If a car gets 60 kilometers per liter of gas, is that good? I have to convert it to miles per gallon to compare it with what I know. It is the same way with units in the carbon cycle. In this book I use the most common units, gigatons of carbon, or Gton C. The prefix giga- means 10⁹, so a gigaton is a billion metric tons. A metric ton is equal to 10⁶ grams, so 1 Gton C = 10¹⁵ g C.

A chemist will tell you that it is more convenient to count atoms than to weigh them. Carbon dioxide is composed of carbon and oxygen atoms in a ratio of 1:2—in atoms, not grams. Chemists count atoms using the unit of mole, where a mole is a number of atoms that will give you the atomic weight of the element in grams. There are 6.02 × 10²³ atoms or molecules in a mole, and mass of a mole of molecules, in grams, is equal to the sum of the atomic masses of the elements. One mole of carbon (12 g) will react with two moles of oxygen (twice 16 g) to make one mole of CO2 (44 g).

One can convert units by writing them out explicitly and canceling them using conversion factors:

1 Gton C × (10¹⁵ g/Gton) × (mol C/12 g) = 8.3 × 10¹² moles,

where the 12 grams in the second factor is the molecular weight of carbon (but only roughly, since there is also some carbon-13 and carbon-14 in natural carbon; see box 2.3, The World According to Carbon Isotopes).

The disadvantage of using mass units is exemplified by the potential for confusion in the climate literature between the mass of carbon alone and the mass of CO2, including the oxygen atoms. Some people talk about the costs of reducing CO2 emissions in dollars per ton of carbon, others in dollars per ton of CO2; the units are very different. The conversion is

1 g C × (44 g CO2/12 g C) = 3.7 g CO2,

where 44 g is the approximate mass of a mole of CO2.

I think of the conversion factor as equivalent to the statement that 44 g CO2 is equal or somehow equivalent to 12 g C. They are different units but equivalent, in the way that 2.54 cm is equivalent to 1 inch. A fraction such as 44 g CO2/12 g C, where the numerator equals the denominator conceptually, divides out to a value of one. You can multiply a number such as 1 g C by the number one, in whatever form, without changing the magnitude of the number, only its units.

Oxygen

The biosphere captures energy from sunlight and stores it in chemical form, maintaining a huge chemical disequilibrium in the biosphere that would not exist on a lifeless planet. This is another example of a second-law-flouting local pocket of order generated by living things. In the 1970s, the geochemist James Lovelock wrote that the chemistry of the biosphere is charged up like a battery and we are machines running off that battery, one pole connected to the oxygen in the atmosphere and the other pole connected to food, the organic carbon produced by photosynthesis (Lovelock 1974).

Box 1.2

Reservoirs of Carbon

CO2 is a trace constituent in the atmosphere, comprising only about 0.039% of all the gas molecules (which works out to 390 ppm, or parts per million). If all the CO2 in the atmosphere were to solidify into dry ice, the snowfall would be only about 10 cm deep. The atmosphere currently contains about 780 Gton C. A handy conversion factor is that 1 Gton C in our atmosphere changes the concentration of CO2 by about 0.5 ppm.

The atmosphere acts as a kind of Grand Central Station with respect to the carbon cycle, and the other reservoirs interact with each other primarily by trading carbon through the atmosphere, even though the atmosphere holds only a tiny fraction of Earth's carbon. The situation is different on Venus, for most of that planet's carbon is found in its atmosphere. The difference between the two planets is liquid water, which enables weathering chemical reactions to take place on Earth. Venus lost its water early on. Water is an essential component of the climate-stabilizing weathering CO2 thermostat mechanism. On Venus, the thermostat is broken.

The land biosphere is the most visible part of the carbon cycle to us, and it holds much more carbon than the ocean biosphere does (trees are much larger than single-celled plankton). The land surface stores carbon in organic form in living things and even more abundantly as the organic carbon remains of plants in soils. Grasslands accumulate a lot of organic carbon in their soils, so that the total amount of carbon per acre is about equal to that in forests, even though the carbon is more obvious in the forests. There is about as much living carbon on land as there is atmospheric carbon, perhaps 500 Gton C.

The amount of carbon attributed to the soil pool depends on how deep the boundary is between soil and the geologic record, where soils might be affected by changes in climate but the carbon below that is out of reach. The usual soil depth is assumed to be one meter, resulting in a soil carbon pool of about 1,500 Gton C as soil carbon, twice as much as is in the atmosphere.

The oceans contain about fifty times more carbon than the atmosphere does, about 38,000 Gton C. Most of the carbon in the ocean is in the inorganic forms: dissolved CO2, carbonic acid (H2CO3), bicarbonate ion (HCO3-), and carbonate ion (CO3=). These chemical forms of carbon are oxidized, just as CO2 is oxidized, rather than being chemically reduced, as organic carbon is. This means that it doesn't take photosynthetic energy to convert carbon from one of the inorganic forms to another; it merely takes a change in the pH, the acidity of the water. Most of the carbon is in the pH-neutral form of bicarbonate, and the concentrations of the other species depend on the pH of the seawater.

Sedimentary rocks contain most of the Earth's carbon, in chemical forms of limestone (CaCO3) and organic carbon, mostly in the form of a random indescribable goo called kerogen. The sediments were originally deposited in water but are now found over most of the surface of the Earth, including the highest mountaintops.

Fossil fuels make up only a small fraction of the buried organic carbon in the Earth, but even so, there is enough carbon to knock the carbon cycle significantly out of whack. Fossil fuel combustion takes carbon that was sleeping in sedimentary rocks and injects it into the atmosphere. Most of the fossil carbon is coal, and of that there is enough to increase the CO2 concentration of the atmosphere to about ten times its natural concentration, if it were all released at the same time.

For further information, consult Chapter 7 in Denman et al. 2006.

The oxygen content in the atmosphere, essentially a measure of how charged up the battery of the biosphere is, also seems to be stabilized by the processes going on in the biosphere. Since the advent of multicellular life forms that leave fossils, 600 million years ago, the air has had about as much oxygen as it has today, occasionally a bit more or less. If oxygen dropped to one-tenth of its present-day concentration, multicellular life would end and the fossils would disappear. And if the oxygen concentration ever increased to ten times its present value, wet wood would burn, and a spark of lightning would ignite an unstoppable fire. Neither of these disasters seems to have happened in Earth's history (Berner 2004).

Oxygen is produced by photosynthesis, the process that also produces the organic matter in our food and in fossil fuels. Most of the organic carbon from plants decomposes eventually, and when it does, it consumes the same amount of oxygen as was produced by photosynthesis. The only way oxygen can be left over to build up in the atmosphere is when the organic carbon escapes degradation by being buried someplace where nothing eats it. The oxygen homeostat is not as clearly understood as the CO2 thermostat, but it probably has to do with the effect of oxygen in the ocean on organic carbon burial (see chapter 3).

Box 1.3

Oxidation and Reduction of Carbon

Oxidation and reduction (abbreviated redox) reactions change the number of electrons possessed by the atoms in the reaction. Here we are concerned with carbon atoms in particular, but many other elements undergo changes in oxidation state, such as oxygen, nitrogen, and iron.

In the bookkeeping of electrons, the difference between the carbon atoms in CO2 and methane (CH4) is in the carbon atoms’ chemical partners. Oxygen is greedy for two of carbon's electrons, which are shared with oxygen in a double bond. When oxygen is combined with carbon, the bookkeeping practice is to assign the carbon a deficit of two electrons. Since each electron has a charge of -1, the carbon atom has a bookkeeping charge of +2, one for each electron. The carbon in CO2 has two oxygen partners, each taking two electrons, leaving the carbon with an oxidation state of +4.

In methane, hydrogen donates one electron each to its chemical partners. The carbon atom in methane has an oxidation state of -4, a negative charge for each electron coming from each of four hydrogens.

A carbon with a positive oxidation state is called oxidized, consistent with the oxygens that make up the CO2 molecule. When the oxidation state is negative, it is called reduced, consistent with the lower number of the oxidation state. Chemists call carbon organic when it is at least partially reduced, as a historical holdover from the early association of reduced carbon with life. Note the contrast with everyday usage: a chemist would call the pesticide DDT a form of organic carbon, but a grocer uses the word to mean that food is grown without using pesticides like DDT.

The organic matter produced when plants grow is complex stuff (see box 1.5, Biochemistry 101), but on average, the carbon atoms have an oxidation state of about zero, similar to a molecule of CH2O.

Ocean pH

The acidity, or pH, of the ocean is also controlled by elements of the carbon cycle, in particular the cycling of CaCO3 between sedimentary rocks and the ocean (Broecker and Takahashi 1978). CaCO3, the chemical constituent of limestone, a very common kind of sedimentary rock, behaves chemically as a base and dissolves in acid. The pH of the ocean is controlled by the dissolved CaCO3 that flows into and out of the ocean. The input is from the dissolution of land rocks, including CaCO3-containing rocks, a geochemical reaction weathering, and the output is in the form of CaCO3 deposition on the seafloor. The ocean pH must be right for sedimentation to balance weathering, and as the ocean balances its CaCO3 budget, it controls the ocean pH. This mechanism, called the CaCO3 pH-stat, takes a few thousand years to adjust the ocean's pH, faster than either the weathering